Polysulfone membranes are known in the art in both flat sheet and hollow fiber forms. These membranes have a wide variety of uses, including desalination by reverse osmosis, dialysis, and protein separations.
A major drawback of using membranes for concentration or fractionation of proteins in biotechnology downstream separation processes is the adsorption of protein molecule solutes on the membrane surface. This has an important impact on the flux decline, usually called "membrane fouling", and the performance characteristics of the membrane change with operating time. As a consequence, the separation process design becomes more difficult to optimize and sometimes requires complex operation modes. The easiest way to overcome this problem is to employ a protein non-adsorptive membrane.
Substantially protein non-adsorptive polysulfone microporous membranes are disclosed in U.S. Pat. No. 5,096,585, issued to Nguyen Mar. 17, 1992, and incorporated herein by reference as if set forth in full. Membranes made according to that reference have high burst strength, high flux and low protein adsorption characteristics suitable for applications to biotechnology, including dialysis. An added feature which would enhance the usefulness of these membranes would be reusability. Multiple hypochlorite (bleach) washes are used for cleaning such membranes and their associated equipment. Other oxidizers such as hydrogen peroxide and peracetic acid, or mixtures thereof, are also used. The membranes do not hold up well to such treatment. As a consequence, their use has thus far been limited to single applications or multiple applications where alternative protocols have been established. It has now been found that if such membranes are made by incorporating certain hydrophilic polymer formulations, the composite membranes exhibit enhanced resistance to bleach. Resistance to bleach is desirable as it extends the useable lifetime of the membranes.